PARIS—Fixing the rules for how developed nations will pay for developing nations to respond to climate change is now the main obstacle to reaching a global deal at a summit here, said Nozipho Mxakato-Diseko, the South African diplomat who is representing a group of more than 100 developing nations in the talks.

Ms. Mxakto-Diseko said that the latest draft of a climate-change deal is too vague regarding who will pay for developing nations to limit their greenhouse-gas emissions and protect themselves from the effects of climate...